MICROMACHINING.ppt (Size: 1.24 MB / Downloads: 1680)
ABSTRACT
Miniaturization is the order of the day. Until recently a decade ago traditionally watch parts were considered to be the micro components one can think off. Recent changes in societyâ„¢s demand have forced us to manufacture variety of micro components used in different fields starting from entertainment electronics to be bio medical implants. Present day manufacturing processes used for miniaturization are the micro electronic fabrication techniques used for Micro Electro Mechanical Systems (MEMS). The limitation of all these processes is that they are applicable for producing 2D patterns and thickness of parts is very low; say a few microns and they are employed on materials such as Silicon and crystalline materials and not metals. Miniaturized parts may have overall sizes of a few millimeters but may have many features that fall in micron range. Also we need many such miniaturized parts may be with 3D profiles, that too made of metals in many fields like aerospace to bio-medical applications. A new candidate requiring micro products is the filed of biotechnology. In the medical field, diagnosis and surgery without pain are achieved through miniaturization of medical tools. Micromachining is one of the key technologies that can enable the realization of all of the above requirements for microproducts and fields with such requirements are rapidly expanding. If complementary machining processes are developed to overcome the above shortcomings, metallic miniature devices will be economically feasible reality.
MICROMACHINING
Literally Micro in micro machining implies that parts are made to the size of 1 to 999 Ã‚Âµm. However Micro also means very small in the fields of machining, manufacture of small parts are not easy. Therefore micro components should also indicate too small components to be machined Prof. Taniguchi defines Micro Engineering as the fields where components sizes are a few millimeters. When the part size is between 100Ã‚Âµm to 100mm, a term MESO manufacturing is also used to address such parts. In fact, the range of micro varies according to era, person, machining method, type of product or material.
Micromachining conditions
The machining processes for micro/meso manufacturing can be derived from traditional machining processes such as turning milling, drilling, grinding, EDM, laser machining, etc., by judicious modification of these machines. Unit metal removal and improving equipment precision are the key factors for adapting the traditional machining processes to micro machining. When these two guidelines are set, the approach is almost correctly directed toward micromachining.
Unit removal
The concept of unit removal was introduced as processing unit by N Taniguchi to explain the difference in removal phenomena between micromachining and conventional machining. Unit removal (UR) is defined as the part of work piece (length, area or volume) removed in one cycle of metal removal operation. Since UR gives the achievable tolerance on the part it should be much smaller than the size of the part it should be much smaller than the size of the component. The smallest UR is the size o0f the atom. UR of sub-micrometer order is also required when the object size is very small or when high precision of the product is required. It is difficult to achieve ideal UR and machine accuracy in the lower range of sizes, say 1 to 10 microns
Equipment precision
When a maniaturised part is required the component is scaled down. Then, it is necessary that the dimensional error of the product be likewise reduced. Therefore, higher precision of the micromachining equipment is desired although it is often impossible to reduce the dimensional error in proportion to the size of product. If the above two equipments small UR and high equipment precision were satisfied micro-machining would be possible independent of the type of machining process. Since the theoretical minimum UR possible in mostprocess are of of the nanometer order. Micro-machining is thoreitically possible in most existing machining progresses on the other hand the theoretical smallest UR is largest than the size of the atom. This suggest that in micromachining in the lower range of dimensions for example. 1 to 10 Ã‚Âµm it mavoe more difficult to achieve the ideal UR and equipment precision because of the influence of this absolute limit.
Classification according to machining phenomena
Â¢ Removal by Mechanical Force
Â¢ Removal by ablation
Â¢ Removal by dissolution
Â¢ Plastic Deformation
The following Table shows the major methods grounded by the machining mechanism and the work material amenable for those process.
Table: Micromachining Processes
Machining phenomena Micromachining Process Materials
Force Micromilling , micro grinding Ceramics, metals,si
Ablation Excimer Laser, Femto Second Laser Ceramics, Polymers
Dissolution Etching, ECM
Reactive ion etching (R:E) Galss, Quartz Si
Ceramics, Polymers
Plastic Deformation. Punching , Press Plastic Deformation
LASER MICROMACHINING
unlike the CO2 or Nd: YAG lasers, Examier and Femto Second lasers, on the contrary, offer high-precision machining without the formation of a re-solidified layer and a heart â€œaffected zone. There are two types of methods that are based on material removal by ablation. One uses a power source that emits a beam with very high quantum energy. If the energy exceeds the binding energy among atoms of the workpiece each molecule can be decomposed directly into atoms and removed from the workpiece. The other method uses an energy beam of which incident power density on the workpiece is extremely high such a high power enables the removal of the workpiece by vaporization, skipping the phase of melting in some cases, molecules are also decomposed in both types, microshapes can be generated by projecting mask patterns, whose size is reduced by using optics. Excimer laser and femto second lasers (hereafter referred to as FS lasers) are respectively typical examples of power sources for the above two types. The Excimer laser is an ultraviolet laser which can be used to micromachine a number of materials without heating them, unlike many other lasers which remove materials without heating them, unlike many other lasers which remove materials without heating them, unlike many other lasers which remove material by burning or vaporising it. Higher accuracy can e achieved when a shorter wavelength, for example, 193nm of an ArF laser is applied. Since the applied photon energy is similar to the energy level of molecular bonds in plastics, the ideal targets for excimer laser machining are plastics, and similar materials and not metals. When a very high power is applied, the removal phenomenon involves a combination of heating and photon attack. FS lasers have short (femto second) pulse duration and high (tera watt) power and overcomes the above limitation. The remarkable feature of these methods is that little heat affected layer remains on the machined surface. This leads to the possibility of machining microshapes with high dimensional accuracy and less defects in the surface layer. The main drawbacks are low efficiency in material removal and consequently, low machining speed another drawback is the high cost of equipment due to their short history.
SETUP
Conceptual Solid Model of Laser Micromachining Setup
WORKING
Laser is emitted from the source is passed through the energy attenuator. After it is passed through the beam homogenizer to homogenize the beam. The target illuminator and machine vision controls the beam to the focusing lens. The lens is moved by precision motion stages. The beam is then falls on the work piece and the machining is takes place.
Characteristics of Femtosecond Laser Micromachining
Â¢ Very high peak powers in the range 1013W/cm2 provide for minimal thermal damage to surroundings
Â¢ Very clean cuts with high aspect ratios
Â¢ Sub-micron feature resolution
Â¢ Minimal redeposition
Â¢ Possible to machine transparent materials like glass, sapphire etc
Ultra short Pulses vs. Long Pulse Micromachining
Femtosecond Laser Micromachining
Micromachining in 18Ã‚Âµm Thick Aluminum Foil
Holes drilled in 25Ã‚Âµm thick brass foil
Ablation Rate vs. Energy Density in 18mm Thick Aluminum Foil
Optimization of Pulse Energy Required to Drill Holes
Automation of Laser Micromachining Process
Application of micromachining
Â¢ Micro milling
Â¢ Micro grinding
Â¢ Chemical etching
Â¢ Micro punching
Â¢ Manufacturing of injection nozzles, Micro surgical tools, VLSI circuits
Micromilling & MicroGrinding
Among the conventional machine processes based on material removal from a workpiece, the most popular case those in which the useless part of the workpiece is removed by mechanical force through plastic or brittle breakage. In the process of this type, the first requirement of micromachining ,small Ur. Is satisfied when a high stress that causes breakage of material is applied to a very small areaor volume of the workpiece. Although cutting is the most conventional machining process, the availability of ultra precision cutting machines with highest level of positioning accuracy, has enabled us to apply this process in micromachining.. Turning, milling and griding are examples of processes of this type. For realising this a tool that was its edge sharpe
Micromilling & Microdrilling is capable of the fabricating holes several tens of micrometers in size for practical applications other types of products such as grooves, cavities and 3D convex shapes may be fabricated when a micro end mill is used instead of a micromill. In such cases, the machining force exerts a larger influence on accuracy because the main direction of the force is perpendicular to the tool axis.
Microgrinding can be applied to the fabrication of micropins and microgrooves, where a grinding wheel with large diameter can be used for such application. The only requirement is to reduce the thickness of the grinding wheel to the required resolution of the product, for example, the width of the grove. The thickness of tens of micrometer order is available so far and correspondingly narrow grooves are reasonable targets of this method. Submicron â€œorder grains of diamond, tungsten carbide or CBN are desirable for realizing good product geometry. The UR of grinding is small because cutting is realized by means of micrograins. However, in the field of micromaching, it is not always a superior method. One of the technological problems is the fact that the tool must be made up of an abrasive and a matrix .when the tool size is very small, the grain size cannot be ignored; this leads to certain difficulties in forming the precise shape of the grinding wheel.
Chemical etching
Chemical or electrochemical dissolution in liquid is also utilized in micromachining. In this type of process, the removal mechanism is based on ionic reaction on the workpiece surface. This leads to very small UR in the direction perpendicular to the surface. The other two dimensions are usually specified by a patterned mask. The advantages in etching besides a small UR are as follows:
Â¢ The machining force is almost zero
Â¢ The surface after machining is free from any damage, residual stress or heat effects
Â¢ The mechanical properties of the workpiece do not influence the removal mechanism
Â¢ In most cases the dissolution phenomenon renders the workpiece surface smooth.
Chemical etching is the process of removing layers of silicon in the atomic dimensional level through chemical reaction between a chemical etchant solution and the exposed silicon surfaces. The bonds between the atoms on the surface and the ones immediately underneath are broken in the process and the surface atoms come out loose. If the etching proceeds predominantly in one direction while the etching does not proceed in the perpendicular direction, then it is called as anisotropic etching. In contrast, if is called isotropic etching. The following are the two types of etching predominantly used.
Punching (plastic Deformation)
As there is neither removal nor addition of material in these processes , UR is meaningless. In order to introduce this method into micromachining, we must be able to manufacture the micropounch and die can be produced by applying appropriate micromachining technologies explained above. Therefore, the realisation of micropouncing depends on the development of a system that ensure easy setting of microtools.
The most remarkable advantage is the production speed in many cases, the machining time is of millisecond order in principle. This indicates the suitability of these processes for mass production . the main issue ----- loss of accuracy is the spring back phenomenon or the partial recovery from deformation after processing. Another issue is the flowability limit of the workpiece material. The flowability is sometimes insufficient to follow the sharp corners of the die/mold . the basic restriction of these processes is that only workpieces softer than the die/mold can be processed.
Conclusion
Technology and Ëœrequest to technologyâ„¢ influence each other. As a result, the front of technology advances as the front of request to technology moves to a higher level. As regards micromachining the dimensions of the product is one of the good indicator of the levels of technology and request. How ever, he level of request from the industry varies widely. The development of technology owes much to the high end of the request. Consequently, the average level of the request is always behind the front of technology.
Reference
1. T Masuzawa, state of the Art of Micromching â€œKey note paper CIRP Annals, vol.49/2/2003, P.473-488
2. R Subramonium, SMK lyer, Micro/ Meso â€œMechanical Manufacturing , National Conference on Advances in Aerospace Manufacturing, Feb. 2002,pp.103-6
3. S Ahmed, Sillicon MEMS Sensor Development, National Conference on Advances in Aerospace Manufacturing , Feb. 2002
4. Dr.K Narayanaswamy â€œ Microfabrication Technology-Proceedings of IPROM 94 â€œBanglore
5. Dr.Rama Bhat.B-Introduction to Mechatronics-proceedings of IPROM 94-Banglore.

Electrochemical Micro-Machining appears to be very promising as a future micro and nano machining technique since in many areas of applications it offers several advantages. In this paper, a review is presented on current research, development and industrial practice of micro-ECM for micro and nano fabrication. New developments in the area of electrochemical micro-machining e.g. micro-electrochemical milling, wire-ECM, solid electrochemical machining, surface structuring and OFLL etc. have also been reported. Future trend of research in the area of utilization of anodic dissolution method for manufacturing of nano range products are also highlighted. The Electrochemical micro machining can effectively be used for high precision machining operations. Further research activities of ECM will open up many challenging opportunities for effective utilization of ECM in the micro machining and nano fabrication domain.

INTRODUCTION

Electrochemical machining (ECM) has seen a resurgence of industrial interest within the last couple of decades due to its many advantages such as no tool wear, stress free and smooth surfaces of machined product and ability to machine complex shape in electrically conductive materials, regardless of their hardness. When this ECM process is applied to the micromachining range for manufacturing ultra-precision shape, it is called electrochemical micro-machining (EMM). Micromachining may literally mean the machining of the dimension between 1 and 999 mm. However, as a technical term, it also means the smaller amount of machining that cannot be achieved directly by a conventional technique. Advanced micro machining may consist various ultra precision activities to be performed on very small and thin work pieces in large numbers. Sometimes, when these things are performed with conventional machining techniques, the problem one usually encounters is high tool wear, rigidity and heat-affected zone [1]. In nonconventional machining most of the machining processes are thermal oriented, e.g. Electro Discharge machining (EDM), Laser beam machining (LBM), Electron beam machining (EBM) etc [2,3]. Chemical machining and Electrochemical machining are thermal free processes, but chemical machining cannot be controlled properly in the micro-machining domain. ECM process is applied to the micro machining range of applications for manufacturing ultra precision shapes; it is called Electrochemical micro machining (EMM). EMM appears to be a very promising micro machining technology due to its advantages that include high MRR, better precision and control, rapid machining time, reliable and environmentally acceptable. In recent years, ECM has received much attention in the fabrication of micro parts [4-8]. Fig.1 shows the schematic view of EMM system set up, which consists of pulsed DC power supply, machine controller, micro tool drive unit, mechanical machining unit and electrolyte flow system etc. Recent research establishes the fact that electrochemical process of metal removal can also be effectively utilized for nano machiningResearch attempts have been made by the electromechanical consumer product industries as well as T.J.Watson research center of IBM apart from other research institutions [7,10]. To exploit full potential of EMM, research is still needed to improve accuracy and compactness. In this paper, a review is presented on current research, development and industrial practice in micro-ECM. An attempt has also been made to report the latest development of ECM in the field of nanofabrication.

Situations where processes are not satisfactory, economical or even possible

High hardness and the strength of the material
Work-piece too flexible
Complex shape
Surface finish and dimensional tolerances
Undesirable Temperature rise and dimensional tolerancesExamples of parts made by advanced Machining Processes
Fig : Examples of parts made by advanced machining processes. These parts are made by advanced machining processes and would be difficult or uneconomical to manufacture by conventional processes. (a) Cutting sheet metal with a laser beam.(b) Microscopic gear with a diameter on the order of 100µm, made by a special etching process.Chemical machining
Chemical attacks metals and etch them by removing small amounts of material from the surface using reagents or etchants

1 – Residual stresses should relieved in order to prevent warping
2 – Surfaces to be thoroughly degreased and cleaned
3 - Masking material(tapes,paints,elastomers & plastics ) is applied
4 – masking is peeled off by scribe and peel technique
5 – The exposed surfaces are etched with etchants
6 – After machining the parts to be thoroughly washed to prevent further reactions with residue etchant
7 – rest of the masking material is removed and the part is cleaned and inspected
8 – additional finishing operations are performed on chemically milled parts
9 – this sequence is repeated to produce stepped cavities and various contours

Process capabilities:
Chemical milling used in the aerospace industry
Tank capacities for reagents are as large as 3.7m x15m
Process also used for micro electronic devices
Surface damage may result due to preferential etching and intergranular attack

Chemical blanking:
Chemical blanking is similar to chemical milling
Material is removed by chemical dissolution rather than by shearing
Burr free etching of printed-circuit boards, decorative panels, thin sheet metal stampings as well as production of small and complex shapes Chemical Machining
Fig : (a) Schematic illustration of the chemical machining process. Note that no forces or machine tools are involved in this process. (b) Stages in producing a profiled cavity by machining; not the undercut.

Negative placed over coated blank and exposed to ultra violet light which hardens the exposed area

Blank is developed which dissolves the exposed areas

Blank is then immersed into a bath of reagent or sprayed with the reagent which etches away the exposed areas

The masking material is removed and the part is washed thoroughly to remove all chemical residues
Design considerations
Designs involving sharp corners,deep & narrow cavities, severe tapers or porous work piece should be avoided

Undercuts may be developed because etchant attacks both in horizontal and vertical direction

To improve production rate the bulk of the work piece should be shaped by other machining process priorly

Dimensional variations can occur ,this can be minimized by proper selection of artwork media by controlling the environment